1. Field of the Invention
The invention relates to the provision of RF shielding for an individual or collection of integrated circuit chip operating at microwave frequencies. The invention is applicable to chips operating in an analog and/or digital mode and deals with the shielding of that integrated circuit chip from interfering fields at microwave frequencies produced by other integrated circuit chips in proximity to the subject chip and contained within a common enclosure.
2. Description of the Prior Art
Integrated circuits operating with analog signals at frequencies in the Gigahertz range are now commonplace. A preferred material for the individual chips is one of the materials having semiconducting properties facilitating high frequency operation such as gallium arsenide. Gallium arsenide has the added advantage of being semi-insulating, allowing conductor runs and passive circuit components to be formed on insulating portions of the surface of the chip without the losses customarily associated with conductor runs formed on silicon or germanium. In fabricating the chips for operation at microwave frequencies both passive and active circuit elements are sufficiently compact for inclusion on the surface of a reasonable sized chip. The active and passive elements, the I/O pads and the interconnections are photolithographically patterned and the resulting device is termed a "monolithic microwave integrated circuit" (MMIC). Because of a variety of fabrication and performance problems, it is usually most convenient to make up higher level circuits performing a higher level function from a plurality of interconnected MMIC chips thus creating a "hybrid" MMIC assembly, which when packaged, is usually termed a "module".
The efficient and low cost interconnection of such MMIC chips becomes a major challenge for successful module performance. Within the modules, which tend to become quite small at the higher frequencies, the individual chips should be interconnected by connections which preserve transmission line quality (i.e. maintain transmission line impedances and avoid reflection causing discontinuities) and which are short to minimize time delays in processing the signal. In addition, if the individual chips are subject to digital controls, a large number of interconnections may be required, which, as the module sizes go down, become more and more closely spaced.
A high density interconnection (HDI) technique has been proposed to meet this challenge. The technique is described in two patents assigned to the Assignee of the present application (U.S. Pat. No. 4,783,695, filed Sept. 26, 1986 entitled "Multichip Integrated Circuit Packaging Configuration and Method"/C. W. Eichelberger and R. J. Wojnarowski and U.S. Pat. No. 4,894,115, filed Feb. 14, 1989 and entitled "Laser Beam Scanning Method for Forming Via Holes in Polymer Materials"/C. W. Eichelberger, R. J. Wojnarowski and K. B. Welles), describes a method using optical patterning of interconnecting MMIC chips requiring the high density of connections adequate to meet the need in such modules.
The HDI process is applicable to high frequency operation where monolithic microwave integrated circuits (MMIC) handling analog signals are involved, to digital circuitry handling digital information, and to mixed microwave and digital circuitry in which the role of the digital circuitry is to control the analog functions.
A common module for mixed analog/digital operation is a microwave transmit/receive module for phased array systems for satellite communication or for radar systems.
The HDI chip interconnection process, as described in the cited patents, conventionally uses a substrate of alumina supporting MMIC chips of gallium arsenide. In the process, the chips are supported in recesses on the alumina substrate with the upper surfaces of both the chips and the substrate flush and with the pads on the chips and adjacent metallization runs on the substrate exposed A thin optically patternable solid dielectric layer is then adhered to the flush surfaces. The dielectric layer bridges small gaps in the underlying surface and accommodates small variations between the heights of the surfaces. Next, "via" holes are etched down through the dielectric to the chip pads and substrate metallizations between which connections are to be made. An optically patterned metallization, as described in the cited patents, is then formed on the dielectric layer which passes down through the vias and interconnects the pads on the chips with the metallizations on the substrate. The process permits successive dielectric layers and successive metallizations to be added for achieving cross-overs, and may be patterned to a high resolution limit. The process provides efficient paths for both microwave signals, digital controls, and DC biases and permits increased package density.
Unfortunately, increasing package density which brings the MMIC chips together, increases stray RF coupling which may prevent one chip from functioning properly in proximity to another. In the particular case of a module for a phased array radar system, high gain circuits operating at low power levels may pick up RF fields from circuits operating at high power levels. Similiary, noise from digitally operated circuits may enter the signal paths and disrupt the operation of the high gain circuits. Stray fields should be prevented from interfering with chip performance. Granted that shielding is an answer, it is highly desirable that it be compatible with the interconnection process which allows increased package density.